The invention relates to an electron gun for generating an electron beam, comprising a cathode unit composed of at least a cylindrical cathode shaft having an end surface which forms the emissive surface. The cathode shaft is surrounded coaxially at least partly by a cylindrical metal heat reflection screen extends around and beyond the open end of the cathode shaft. The cathode shaft and the heat reflection screen are secured in a cathode support.
The invention also relates to a cathode-ray tube comprising such an electron gun.
Such electron guns are used in black-and-white and colour display tubes for television and for data display, in camera tubes and other tubes in which an electron beam is to be generated.
A cathode unit of the construction described in the opening paragraph is disclosed in German Patent Application Ser. No. 1,764,047 laid open to public inspection. The construction described in this Patent Application comprises a cathode shaft which near its open end is secured to a heat reflection screen by means of spot-welding. This heat reflection screen surrounds the cathode shaft coaxially and extends beyond the open end thereof. The heat reflection screen in turn is secured in a cathode support by means of metal strips. As a result of the reflection of heat originating from the cathode shaft back to said cathode shaft by means of a heat reflection screen which preferably is polished on its inside, the efficiency of the cathode is increased.
However, such a construction also has a few disadvantages. Because the cathode shaft is secured near the open end of the heat reflection screen, the emissive surface will during warm-up of the cathode shaft and the resulting thermal expansion, move in the axial direction, which is not desired, as will be explained hereinafter. Moreover, heat will flow from the cathode shaft to the heat reflection screen via the spot-welds and will warm-up said screen. The heat radiated by the heat reflection screen on its outside reduces the efficiency of the system. The warm-up time of the cathode is long as a result of the large heat capacity of the system.